Field of the Invention
The present invention relates to a single-focus optical system and an optical apparatus using the same.
Description of the Related Art
As a type of an image forming optical system having a high imaging performance, a Gaussian type has been known. A Gaussian type optical system includes in order from an object side, an object-side unit having a positive refractive power and an image-side unit having a positive refractive power.
The object-side unit includes two to three positive lenses and one negative lens. This negative lens is cemented to the positive lens immediately before the negative lens in some cases. Moreover, the image-side unit includes one negative lens and two to three positive lenses. This negative lens also, is cemented to the positive lens immediately before the negative lens in some cases.
Moreover, in the Gaussian type optical system, a shape on the object side and a shape on an image side is, by and large, symmetrical about a central portion thereof, and moreover, the Gaussian type optical system has a concentric shape. In the concentric shape, a center of curvature of two lens surfaces in each lens is positioned near the central portion.
Accordingly, in the Gaussian type optical system, even with a large aperture ratio, each aberration is corrected favorably to some extent. However, when an attempt is made to realize an imaging performance superior to that of the conventional one, in the Gaussian type optical system, an angle of view slightly smaller than 50 degrees is the limit. When an attempt is made to realize the angle of view not smaller than 50 degrees in the Gaussian type optical system, in particular, correction of a spherical aberration and a coma becomes difficult. Moreover, in an optical system based on the Gaussian type, the optical system is susceptible to be large-sized.
On the other hand, from a viewpoint that an F-number is small, apart from the Gaussian type, Sonnar type and Ernostar type have been known as types of image forming optical system. Since a back focus is susceptible to become short in these types, adopting an optical system of these types is advantageous from a point of shortening an overall length of the optical system. However, these types have a drawback that it is not possible to widen the angle of view easily.
Various types of wide-angle taking lenses in which these issues are solved, have been proposed. In the wide-angle taking lens that has been proposed, an F-number is about 1.4. As examples of an optical system of wide-angle taking lens with a wide angle of view and a small F-number, optical systems disclosed in Japanese Patent Application Laid-open Publication No. 2012-226309, Japanese Patent Application Laid-open Publication No. 2004-101880, Japanese Patent Application Laid-open Publication No. 2009-109723, Japanese Patent Application Laid-open Publication No. 2010-039340, Japanese Patent Application Laid-open Publication No. 2010-097207, and Japanese Patent Application Laid-open Publication No. 2011-059290 are known.